Refractory product having high zirconia content

ABSTRACT

The present invention relates to a molten, fluid refractory product comprising, in weight percentages on the basis of the oxides and for a total of 100% of the oxides:
         ZrO 2 +Hf 2 O: remainder to 100%   4.5%&lt;SiO 2 &lt;6.0%   Al 2 O 3 &lt;0.80%   0.3%&lt;B 2 O 3 &lt;1.0%   Ta 2 O 5 +Nb 2 O 5 &lt;0.15%   Na 2 O+K 2 O&lt;0.1%   K 2 O&lt;0.04%   CaO+SrO+MgO+ZnO+BaO&lt;0.4%   P 2 O 6 &lt;0.05%   Fe 2 O 3 +TiO 2 &lt;0.55%   other oxide species, optionally including Y 2 O 3 : &lt;1.5%, where Y 2 O 3 &lt;0.3%,
 
the “A/B” ratio of Al 2 O 3 /B 2 O 3  content by weight being 0.5 to 2.0. The invention can be used in glass melting furnaces.

FIELD OF TECHNOLOGY

The invention relates to a new fused cast refractory product having ahigh zirconia content.

PRIOR ART

Among refractory products, a distinction is made between fused castproducts, well known for the construction of glass melting furnaces, andsintered products.

In contrast to sintered products, the fused cast products most oftencomprise an intergranular vitreous phase binding crystalline grains. Theproblems posed by sintered products and by fused cast products, and thetechnical solutions adopted for solving them, are therefore generallydifferent. A composition developed for making a sintered product istherefore not usable as such a priori for making a fused cast product,and vice versa.

The fused cast products, often called electrocast products, are obtainedby melting a mixture of suitable raw materials in an electric arcfurnace or by any other technique that is suitable for these products.The molten material is then cast in a mold, and the product obtainedundergoes a controlled cooling cycle so that it reaches room temperaturewithout cracking. This operation is called “annealing” by a personskilled in the art.

Among the fused cast products, the electrocast products with highzirconia content, i.e. having more than 85 wt. % of zirconia (ZrO₂),have a reputation for their quality of very high corrosion resistancewithout coloration of the glass produced and without generating defects.

Conventionally, the fused cast products with high zirconia content alsocomprise sodium oxide (Na₂O) to prevent the formation of zircon from thezirconia and silica present in the product. The formation of zircon isin fact harmful since it is accompanied by a decrease in volume of theorder of 20%, thus creating mechanical stresses that cause cracking.

The product ER-1195 produced and marketed by Saint-Gobain SEFPRO andcovered by patent EP-B-403 387 is now widely used in glass meltingfurnaces. Its chemical composition comprises about 94% zirconia, 4 to 5%silica, about 1% alumina, 0.3% sodium oxide and less than 0.05 wt. %P₂O₅. It is typical of products with high zirconia content used forglass furnaces.

FR 2 701 022 describes fused cast products with high zirconia contentthat contain 0.05 to 1.0 wt. % of P₂O₅ and 0.05 to 1.0 wt. % of boronoxide B₂O₃. These products have a high electrical resistivity.Advantageously, this makes it possible to stabilize the electric powerconsumption during electric melting of glass and in particular avoid anyproblem of short circuiting in the refractory products leading to theirrapid degradation. In fact, during electric melting of glass, some ofthe electric current passes through the refractory products. Theincrease in the resistivity of these refractory products therefore makesit possible to reduce the amount of electric current able to passthrough them.

WO 2009 027610 describes fused cast products with high zirconia contenthaving a high electrical resistivity in the presence of at least oneoxide selected from Nb₂O₅ and Ta₂O₅ for silica contents between 6 and12%.

WO 2007 099253 describes fused cast products with high zirconia contenthaving a high electrical resistivity in the presence of at least oneoxide selected from CrO₃, Nb₂O₅, MoO₃, Ta₂O₅ and WO₃. These productscontain less than 1.5 wt. % of B₂O₃ for alumina contents between 0.1 and2.4%.

WO 2005 068393 describes fused cast products with high zirconia contenthaving a high electrical resistivity while minimizing the contents ofBaO, SrO, MgO, CaO, P₂O₅, Na₂O and K₂O. These products contain 0.1 wt. %to 1.2 wt. % of B₂O₃ and between 0.8% and 2.5% of alumina, but noattention is drawn to the Al₂O₃/B₂O₃ ratio. Further, none of theexamples in this document includes an alumina content of less than0.85%; an alumina content of greater than 0.9% is preferred.

JP 63 285173 describes fused cast products with high zirconia contenthaving a good electrical resistivity and resistance to cracking forsilica contents less than 6.5%.

The current development of glasses of very high quality, in particularglasses for flat screens of the LCD type, increases the requirements forthe refractory products of glass melting furnaces. In particular, thereis a need for refractory products with improved electrical resistivity,without having to use dopants, while having resistance to corrosion byglass.

The present invention aims to satisfy this need.

SUMMARY OF THE INVENTION

More particularly, the invention relates to a fused cast refractoryproduct comprising, in percentages by weight based on the oxides and fora total of 100% of the oxides:

-   ZrO₂+Hf₂O: complement to 100%-   4.5%<SiO₂<6.0%-   Al₂O₃<0.80%-   0.3%<B₂O₃<1.0%-   Ta₂O₅+Nb₂O₅<0.15%-   Na₂O+K₂O<0.1%-   K₂O<0.04%-   CaO+SrO+MgO+ZnO+BaO<0.4%-   P₂O₅<0.05%-   Fe₂O₃+TiO₂<0.55%-   other oxide species, including optionally Y₂O₃ (when Y₂O₃ is    present, it is counted among said “other oxide species”): <1.5%,    with Y₂O₃<0.3%,    the ratio “A/B” of the contents by weight Al₂O₃/B₂O₃ being between    0.5 and 2.0.

As will be seen later, surprisingly, the inventors discovered that thiscomposition allows the refractory product according to the invention tohave a good electrical resistivity and a good corrosion resistance, eventhough the content of dopant Ta₂O₅ and/or Nb₂O₅ is less than 0.15%.

A refractory product according to the invention can further comprise oneor more of the following optional characteristics:

-   -   The ratio A/B of the contents by weight Al₂O₃/B₂O₃ is less than        or equal to 1.75, preferably less than or equal to 1.6,        preferably less than 1.5, preferably less than 1.2, less than        1.1, or even less than 1.0.    -   The ratio A/B of the contents by weight Al₂O₃/B₂O₃ is greater        than 0.55, or even greater than 0.60.    -   The content by weight of ZrO₂+HfO₂ is less than 95.5%, or even        less than 95.0%, or even less than 94.0% and/or greater than        90.0%, or greater than 91.0%, or even greater than 92.0%.    -   The content by weight of silica SiO₂ is greater than 4.6%,        greater than 4.8%, greater than 4.9%, and/or less than 5.5%.    -   The content by weight of Al₂O₃ is greater than 0.2%, or even        greater than 0.3%, or even greater than 0.35%.    -   The content by weight of alumina Al₂O₃ is less than 0.75%, less        than 0.70%, less than 0.60%, less than 0.55%, or even less than        0.50%.    -   Alumina Al₂O₃ is only present as impurities.    -   The content by weight of B₂O₃ is greater than 0.35%, or even        greater than 0.40%, or even greater than 0.45%.    -   The content by weight of B₂O₃ is less than 0.90%, or even less        than 0.80%, or even less than 0.70%, or less than 0.60%, or less        than 0.55%.    -   The content by weight of (Ta₂O₅+Nb₂O₅) is less than 0.10%, less        than 0.05%, or is even virtually zero.    -   The content by weight of (Na₂O+K₂O) is less than 0.04.    -   The oxides of iron and/or of titanium and/or of calcium and/or        of strontium and/or of barium and/or of magnesium and/or of zinc        and/or of phosphorus are only present as impurities.    -   The content by weight of oxides of iron and/or of titanium,        Fe₂O₃+TiO₂, is less than 0.4%, preferably less than 0.3%,        preferably less than 0.2%.    -   The content by weight of oxide of calcium and/or of strontium        and/or barium and/or of magnesium and/or of zinc is less than        0.2%, preferably less than 0.1%.    -   The total content by weight of oxides of calcium and/or of        strontium and/or barium and/or of magnesium and/or of zinc,        CaO+SrO+BaO+MgO+ZnO, is less than 0.3%, preferably less than        0.2%, less than 0.1%, less than 0.05%.    -   The total content by weight of the “other oxide species” is less        than 1.0%, less than 0.6%, less than 0.5%, or even less than        0.3%.    -   The “other oxide species” only consist of impurities and the        total content by weight of the “other oxide species” is less        than 0.6%, less than 0.5%, or even less than 0.3%.    -   The content by weight of yttrium oxide Y₂O₃, which forms part of        the “other oxide species” is less than 0.25%.    -   The content by weight of SnO₂+CuO is less than 0.05%.

According to a particular embodiment, the invention proposes a fusedcast refractory product comprising, in percentages by weight based onthe oxides:

-   92.0%<ZrO₂+Hf₂O<95.0%-   4.5%<SiO₂<6.0%-   Al₂O₃<0.55%-   0.35%<B₂O₃<0.55%-   Ta₂O₅+Nb₂O₅<0.05%-   Na₂O+K₂O<0.04%-   CaO+SrO+MgO+ZnO+BaO<0.2%-   other oxide species (other than ZrO₂, Hf₂O, SiO₂, Al₂O₃, B₂O₃,    Ta₂O₅, Nb₂O₅, Na₂O, K₂O, CaO, SrO, MgO, ZnO, and BaO): complement to    100%,    the ratio “A/B” of the contents by weight Al₂O₃/B₂O₃ being between    0.6 and 1.2.

Preferably, the other oxide species represent less than 2.5%, less than2.0%, less than 1.5%, less than 1.0%, less than 0.5%, or even less than0.3%.

The invention also relates to a method for manufacturing a refractoryproduct according to the invention, comprising the following successivesteps:

-   -   a) mixing raw materials so as to form an initial charge,    -   b) melting said initial charge until a molten material is        obtained,    -   c) casting and solidification of said molten material, by        cooling, so as to obtain a refractory product,        said method being characterized in that said raw materials are        selected so that said refractory product is according to the        invention.

Preferably, oxides for which a minimum content is required, notablyZrO₂, SiO₂, B₂O₃ or precursors of these oxides, are added systematicallyand methodically. Preferably, the contents of these oxides in thesources of the other oxides, where they are conventionally regarded asimpurities, are taken into account.

Preferably, cooling is controlled, preferably so as to be carried out ata rate of less than 20° C. per hour, preferably at a rate of about 10°C. per hour.

The invention also relates to a glass melting furnace comprising arefractory product according to the invention, or a refractory productmanufactured or that can be manufactured by a method according to theinvention, in particular in a region intended to be in contact withmolten glass. In a furnace according to the invention, the refractoryproduct can advantageously form part of a tank for making glass bymelting, notably by electric melting, where it may come in contact withmolten glass at a temperature greater than 1200° C.

DEFINITIONS

The contents of oxides by weight refer to the total contents for each ofthe corresponding chemical elements, expressed in the form of the moststable oxide, according to the convention that is usual in the industry;therefore this includes the suboxides and optionally nitrides,oxynitrides, carbides, oxycarbides, carbonitrides, or even the metallicspecies of the aforementioned elements.

A “molten material” is a liquid mass which, to conserve its shape, mustbe contained in a container. It can contain some solid particles, but ofan insufficient amount to be able to structure said mass.

“Impurities” means the inevitable constituents, introducedunintentionally and necessarily with the raw materials or resulting fromreactions with these constituents. The impurities are not necessaryconstituents, but are merely tolerated. For example, the compoundsincluded in the group of the oxides, nitrides, oxynitrides, carbides,oxycarbides, carbonitrides and metallic species of iron, titanium,vanadium and chromium are impurities.

“Dopant” here means the constituents Ta₂O₅ and/or Nb₂O₅.

Unless stated otherwise, all the contents of oxides in the productsdescribed and claimed are percentages by weight based on the oxides.

DETAILED DESCRIPTION OF THE INVENTION

In a fused cast product according to the invention, high content ofzirconia ZrO₂ makes it possible to meet the requirements of highcorrosion resistance without coloration of the glass produced andwithout producing defects that impair the quality of said glass.

In a product obtained by melting, HfO₂ cannot be dissociated chemicallyfrom ZrO₂. In the chemical composition of such a product, ZrO₂+HfO₂therefore denotes the total content of these two oxides. However,according to the present invention, HfO₂ is not added deliberately inthe initial charge. HfO₂ therefore only denotes traces of hafnium oxide,this oxide always being naturally present in sources of zirconia atcontents generally less than 2%. For clarity, the content of zirconiaand traces of hafnium oxide can therefore be denoted either by ZrO₂+HfO₂or by ZrO₂, or by “zirconia content”.

The content of hafnium oxide HfO₂ in a product according to theinvention is less than 5%, generally less than 2%.

The presence of silica SiO₂ notably permits the formation of anintergranular vitreous phase that can effectively accommodate thechanges in volume of the zirconia during its reversible allotropictransformation, i.e. during transition from the monoclinic phase to thetetragonal phase.

However, addition of silica must be limited in order to obtain highcorrosion resistance. Moreover, too high a content of silica might causedefects in the glass through detachment of stones (pieces of refractoryproduct resulting from loss in cohesion of the product), which isregarded as poor behavior in application.

The presence of alumina promotes the formation of a stable vitreousphase and improves the castability of the products in the mold. Anexcessive content leads to instability of the vitreous phase (formationof crystals), which has an adverse effect on feasibility, in particularin the presence of boron oxide. The content by weight of alumina musttherefore remain limited.

The presence of a content by weight of B₂O₃ in a proportion such thatthe ratio A/B of the contents by weight Al₂O₃/B₂O₃ is less than or equalto 2.0 makes it possible to increase the electrical resistivity.

The oxides Na₂O and K₂O have an adverse influence on electricalresistivity. The content by weight of Na₂O+K₂O must therefore be lessthan 0.1%. In particular, the content of K₂O must be less than 0.04%.

According to the invention, the content by weight of Fe₂O₃+TiO₂ is lessthan 0.55% and that of P₂O₅ is less than 0.05%. In fact, these oxidesare known to be harmful and their content must preferably be limited totraces introduced as impurities with the raw materials.

Unless stated otherwise, the “other oxide species” are the species thatare not listed above, namely species other than ZrO₂, Hf₂O, SiO₂, Al₂O₃,B₂O₃, Ta₂O₅, Nb₂O₅, Na₂O, K₂O, CaO, SrO, MgO, ZnO, BaO, P₂O₅, Fe₂O₃ andTiO₂. In one embodiment, the “other oxide species” are limited tospecies whose presence is not particularly desired and which aregenerally present as impurities in the raw materials.

Conventionally, in a fused cast product, the oxides represent more than98.5%, more than 99%, or even roughly 100% of the mass of the product.It is the same in a product according to the invention.

A product according to the invention can be manufactured conventionallyaccording to steps a) to c) described less than:

-   -   a) mixing raw materials so as to form an initial charge,    -   b) melting said initial charge until a molten material is        obtained,    -   c) solidification of said molten material, by cooling, so as to        obtain a refractory product according to the invention.

In step a), the raw materials are selected so as to guarantee thecontents of oxides in the finished product.

In step b), melting is preferably effected through the combined actionof a fairly long electric arc, not causing reduction, and mixing whichpromotes reoxidation of the products.

To minimize the formation of nodules of a metallic appearance andprevent the formation of cracks or crazing in the final product, it ispreferable to perform melting in oxidizing conditions.

The long-arc melting process described in French patent No. 1 208 577and its additions No. 75893 and 82310 is preferably used.

This process consists of using an arc furnace whose arc is struckbetween the charge and at least one electrode some distance from saidcharge and adjusting the length of the arc so that its reducing actionis minimized, while maintaining an oxidizing atmosphere above the moltenbath and mixing said bath, either by the action of the arc itself, or bybubbling an oxidizing gas (air or oxygen, for example) in the bath or byadding substances that release oxygen, such as peroxides or nitrates, tothe bath.

Melting can in particular take place at a temperature greater than 2300°C., preferably between 2400° C. and 2500° C.

In step c), cooling is preferably carried out at a rate of less than 20°C. per hour, preferably at a rate of about 10° C. per hour.

A product of the invention manufactured in this way consists of grainsof zirconia surrounded by a vitreous phase. The zirconia can bemonoclinic to more than 80%, more than 90%, more than 99% or roughly100%, as percentage by weight. The vitreous phase can comprise more than50%, or even more than 70%, of silica, between 5% and 20% of B₂O₃ andbetween 1% and 20% of alumina, in percentages by weight based on thevitreous phase. The silica, B₂O₃ and alumina can represent more than95%, more than 97%, or even roughly 100% of the weight of the vitreousphase.

Any conventional method of manufacturing fused products based onzirconia intended for applications in glass melting furnaces can beemployed, provided that the composition of the initial charge allowsproducts to be obtained having a composition according to that of aproduct according to the invention.

EXAMPLES

The following non-limiting examples are given for the purpose ofillustrating the invention.

In these examples, the following raw materials were used:

-   -   zirconia containing mainly, on average by weight, 98.5%        ZrO₂+HfO₂, 0.2% SiO₂ and 0.02% Na₂O,    -   zircon sand at 33% silica,    -   boron oxide of purity greater than 99%.

The raw materials were melted by the conventional process of arc furnacemelting and then the molten material was cast to obtain blocks with thedimensions 220 mm×450 mm×150 mm.

Example 1 corresponds to the product ER1195, marketed by Saint-GobainSEFPRO, and constitutes the reference.

For all the products obtained, crystallographic analysis revealscrystals of monoclinic zirconia surrounded by a vitreous phase typicallyhaving more than 70% of silica. All of the silica as well as the otheroxide species with the exception of zirconia are in the vitreous phase.

The chemical analysis of the products obtained is given in Table 1; thisis an average overall chemical analysis, given in percentages by weight.

In the following Table 1, * indicates that the example is outside of theinvention, and an empty cell corresponds to a content less than or equalto 0.05 wt. %.

Cylindrical bars of product 30 mm in diameter and 30 mm high wereextracted from the various examples of blocks produced. These bars weresubmitted to a potential difference of 1 volt at a frequency of 100hertz at 1500° C. or 1600° C. respectively for measuring the electricalresistivity, “R1500” and “R1600” respectively.

TABLE 1 Compositions by weight (as percentage based on oxides) ZrO₂ SiO₂B₂O₃ Al₂O₃ Na₂O A/ R1500 R1600 (%) (%) (%) (%) (%) B (Ω · cm) (Ω · cm)1* Com- 4.0 1.20 0.3 70 2* plement 4.5 0.40 0.80 2.0 110 80 3* to 4.10.40 0.61 1.5 107 81 4  100% 5.0 0.52 0.50 1.0 142 123 5  5.1 0.52 0.400.8 160 132

The results show that the products of the invention tested haveremarkable electrical resistivity despite the absence of dopant(Ta₂O₅+Nb₂O₅). In particular, all the inventive examples exhibitperformances very largely superior to those of the product ER1195(example 1).

Comparison of examples 2* and 3* shows that a decrease in the NB ratioof between 2.0 and 1.5 has no positive effect if the silica content isless than or equal to 4.5%.

Comparison of examples 3* and 4 shows, conversely, that a decrease inthe A/B ratio of between 1.5 and 1.0 accompanied by an increase in thesilica content of between 4.1% and 5.0% has a very positive effect.

Example 3* also shows that excessively low silica contents limitperformances.

Comparison of examples 4 and 5 shows the positive effect of the decreasein the A/B ratio to between 1.0 and 0.8.

Finally, measurements showed that the resistance to corrosion by moltenglass of the products of the invention is equivalent to that of thereference example 1*.

Of course, the present invention is not limited to the embodiments inthe examples, which are given for purposes of illustration.

The invention claimed is:
 1. A fused cast refractory product comprising,in percentages by weight based on the oxides and for a total of 100% ofthe oxides: ZrO₂+Hf₂O: complement to 100% 4.5%<SiO₂<6.0% Al₂O₃<0.80%0.3%<B₂O₃<1.0% Ta₂O₅+Nb₂O₅<0.15% Na₂O+K₂O<0.1% K₂O<0.04%CaO+SrO+MgO+ZnO+BaO<0.4% P₂O₅<0.05% Fe₂O₃+TiO₂<0.55% other oxidespecies, optionally including Y₂O₃: <1.5%, with Y₂O₃<0.3% the ratio“A/B” of the contents by weight Al₂O₃/B₂O₃ being between 0.5 and 2.0. 2.The product as claimed in claim 1, wherein the ratio A/B is less than1.5.
 3. The product as claimed in claim 1, wherein Nb₂O₅+Ta₂O₅<0.10%. 4.The product as claimed in claim 3, wherein Nb₂O₅+Ta₂O₅<0.05%.
 5. Theproduct as claimed in claim 1, wherein the content by weight of silicaSiO₂ is greater than 4.8%.
 6. The product as claimed in claim 1, whereinthe content by weight of B₂O₃ is less than 0.80%.
 7. The product asclaimed in claim 6, wherein the content by weight of B₂O₃ is less than0.55%.
 8. The product as claimed in claim 1, wherein the content byweight of Al₂O₃ is less than 0.70%.
 9. The product as claimed in claim8, wherein the content by weight of Al₂O₃ is less than 0.55%.
 10. Theproduct as claimed in claim 1, wherein the content by weight of Y₂O₃ isless than 0.25%.
 11. The product as claimed in claim 1, wherein thecontent by weight of Na₂O+K₂O is less than 0.04%, the content by weightof oxides of iron and/or of titanium, Fe₂O₃+TiO₂, is less than 0.4%, thecontent by weight of P₂O₅ is less than 0.05%, the total content byweight of oxides of calcium and/or of strontium and/or of barium and/orof magnesium and/or of zinc, CaO+SrO+BaO+MgO+ZnO, is less than 0.3%, andthe total content by weight of “other oxide species” is less than 0.6%.12. The product as claimed in claim 1, comprising, in percentages byweight based on the oxides: 92.0%<ZrO₂+Hf₂O<95.0% 4.5%<SiO₂<6.0%Al₂O₃<0.55% 0.35%<B₂O₃<0.55% Ta₂O₅+Nb₂O₅<0.05% Na₂O+K₂O<0.04%CaO+SrO+MgO+ZnO+BaO<0.2% oxide species other than ZrO₂, Hf₂O, SiO₂,Al₂O₃, B₂O₃, Ta₂O₅, Nb₂O₅, Na₂O, K₂O, CaO, SrO, MgO, ZnO, and BaO:complement to 100%, the ratio “A/B” of the contents by weight Al₂O₃/B₂O₃being between 0.6 and 1.2.
 13. The product as claimed in claim 1,wherein the content by weight of Al₂O₃ is less than 0.50%.
 14. Theproduct as claimed in claim 1, wherein the ratio A/B is less than 1.1.15. A glass melting furnace comprising a product as claimed in claim 1in a region intended to be in contact with molten glass.